There exists a large allocated bandwidth around the 60 GHz region of the electromagnetic spectrum, offering the appeal of high-speed short distance wireless personal area networks (WPANs), radar applications such as automotive radar, along with other potential industrial, scientific and medical applications. This has raised interest in low cost, high efficiency and small form factor integrated millimeter-wave devices in order to facilitate their use in consumer electronic applications. Wireless systems operating at such millimeter-wave frequencies require appropriate antennas and RF components.
Bandpass RF filters are critical for modern wireless communication systems. The filter ensures that the communication system does not transmit power in frequencies that are used by other users or prohibited by regulatory authorities. In order to achieve increasingly higher data rates modern high speed wireless communication systems use complex modulation schemes such as orthogonal frequency division multiplexing (OFDM). Out of band emissions are particularly problematic for OFDM systems where the high peak to average ratio occasionally pushes the transmit power amplifier into compression that generates, if unfiltered, outputs harmonics of the input signal and consequently high out-of-band spectral content. At lower frequencies, system designers and RF engineers include external bandpass filters to ensure the transmit power spectral density mask meets regulatory requirements. Unfortunately external bandpass filters are expensive and the transition from chip to the printed circuit board mounted filter usually degrades the signal.
As communication systems move to millimeter wave frequencies the physical dimensions of RF components becomes smaller than the usual size of a CMOS die, making it theoretically possible to have most of the wireless transceiver implemented on a single CMOS die, which motivates the development of system on chip or system in a package. CMOS is a standard and low cost process for building digital circuits, but CMOS active filters are unidirectional, suffer from distortion at high power and increase noise figure. To date, designs have mostly avoided fabricating passive on-chip filters on standard CMOS technology, because of the lossy conductive nature of the silicon substrate, poor performance, low quality factor (Q) of the resonators in filters, unstable performance due to relatively large fabrication variation, and stringent foundry fabrication design rules. Most integrated passive filters are thus built on high-resistivity substrate materials, however these raise costs.
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